What are the classifications of raw water for water treatment?

Author: Hongjie Water

In different situations, there are different types of water and significant differences in its water quality.

1. Groundwater and surface water 

Groundwater - has less organic and microbial pollution, while calcium magnesium plasma dissolves more and has higher hardness, making it prone to scaling; Sometimes iron/manganese/fluoride ions exceed the standard and cannot meet the demand for production and domestic water.

Surface water - has more organic and microbial pollution than groundwater. If the area belongs to limestone areas, its surface water often has higher hardness, such as Deyang, Mianyang, Guangyuan, Aba and other areas in Sichuan. 

2. Hard water and soft water 

Hard water - The total concentration of metal ions such as calcium and magnesium in water is called hardness. Hard water has a significant impact on production water such as boilers, and it should be softened/desalinated. Hardness greater than 200mg/L is usually referred to as hard water. 

Soft water - water with lower hardness.

3. Raw water and purified water

Raw water - usually refers to the inflow of water treatment equipment, such as commonly used urban tap water (internationally known as domestic drinking water)/suburban groundwater/outdoor surface water, etc. The water quality is often tested with a TDS value (total dissolved solids content in water). The TDS value of Chinese urban tap water is usually 100-400ppm. 

Purified water - Raw water is called purified water after being treated by water treatment facilities. 

4. Purified water and distilled water

Pure water - pure water that can be directly consumed after being treated with a complete set of water treatment facilities such as reverse osmosis and sterilization devices, removing the vast majority of inorganic salt ions, microorganisms, and organic impurities in the raw water. 

Distilled water - Pure water prepared by distillation, usually not used for drinking. 

5. Pure water and Ultrapure water

Pure water - deionized water prepared by methods such as reverse osmosis, distillation, and ion exchange, with a TDS value typically<5PPm and a conductivity typically<10 μ S/cm (resistance value>0.1M Ω. cm). 

Ultrapure water - purified and deionized by means of ion exchange, distillation, electric desalting, etc. Its TDS value is not measurable, and its conductivity is usually<0.1 μ S/cm (resistance value>10M Ω. cm), its ions are almost completely removed. The theoretical pure water resistance value is 18.25 M Ω. cm.

6. Purified water and injection water 

Purified water - Pure water used in the pharmaceutical industry is called purified water, and the conductivity is usually required to be<2 μ S/cm. 

Injection water - Purified water is purified by multi effect distillation/ultrafiltration to remove pyrogen and used to prepare injection. 

Raw water type and water quality analysis. The composition of raw water is an important basic information for determining suitable water treatment processes, selecting suitable water quality treatment processes, and calculating water treatment equipment.

1. PH value

The pH value of raw water reflects its acidity and alkalinity. PH=7 is neutral; PH=0-7 is acidic; PH=7-14 is alkaline. The change in pH affects the ion removal rate, which is the change in the desalination rate of the system. 

2. Conductivity, Total dissolved solids (TDS) 

Conductivity is an indicator of the conductivity of dissolved ions in water. Conductivity is a convenient method for measuring ion concentration in water, but it cannot reflect the composition of ions. The conductivity increases with the increase of ion concentration.

TDS (Total Dissolved Solid Content) is the remaining inorganic matter after filtering out suspended solids and evaporating all water from colloids. The TDS value can be directly measured with a measuring instrument or converted into a TDS value by measuring the conductivity of water. 

3. Hardness 

The hardness of water refers to the concentration of calcium and magnesium ions in the water, and the hardness unit is mg/L measured in Caco3. For water sources with high hardness and alkalinity, special attention should be paid to preventing Caco3 scaling in the pre-treatment process.

4. Turbidity 

Turbidity refers to the degree to which suspended solids in water hinder the transmission of light. Suspended solids in water are generally soil, sand particles, fine organic and inorganic matter, plankton, microorganisms, and colloidal substances.

5. Ionic composition

Inorganic salts dissolved in water, which form insoluble or slightly soluble salts after the combination of anions and cations, have a certain solubility at a certain temperature. In the RO system, they are continuously concentrated with water inflow, and when their solubility limit is exceeded, they will scale on the RO membrane surface. Common insoluble salts are CaCO3 and CaSO4, while other compounds that may be produced are CaF2, BaSO4, etc. If the anions and cations in the water can form the above insoluble or slightly soluble salts, the pre-treatment must consider scaling control measures to prevent the insoluble or slightly soluble salts from exceeding their solubility and causing precipitation and scaling.

6. Alkalinity 

Alkalinity refers to the content of alkaline substances in water that can react with strong acids. The alkalinity mainly represents the content of bicarbonate, carbonate and Hydroxide ion ions in water, which is divided into phenolphthalein alkalinity and total alkalinity.

Different types of water sources correspond to different pre-treatment processes and different types of membrane components. For small engineering projects that do not have water quality analysis, the pre-treatment design can be based on projects that have already been put into operation with the same type of water source. However, for large engineering projects, comprehensive water quality analysis must be conducted.